Hemostasis is a vital function that stops bleeding and protects the integrity of blood circulation on both molecular and macroscopic levels. Hemostasis includes a coagulation cascade of sequentially activatable enzymes that is traditionally divided into three parts: 1) an intrinsic pathway, which includes interactions of blood coagulation proteins that lead to the generation of coagulation factor IXa without involvement of coagulation factor VIIa; 2) an extrinsic pathway, which includes interactions of blood coagulation proteins that lead to the generation of coagulation factor Xa and/or IXa without involvement of thromboplastin antecedent (factor XI); and 3) a common coagulation pathway, including interactions of blood coagulation proteins II, V, VIII, IX and X that lead to the generation of thrombin (factor IIa). Thrombin activates platelets and generates fibrin, both of which are essential building elements of the hemostatic plug that is responsible for sealing the vascular breach. Complete absence of thrombin or platelets causes paralysis of hemostasis and leads to lethal hemorrhage.
Thrombosis, like hemostasis, is a platelet and thrombin dependent process. Thrombosis is a pathological, intravascular, thrombin-dependent, progressive deposition of polymerized fibrin and activated platelets that causes occlusion of blood vessels in various organs. In healthy mammals, intravascular coagulation is localized to the site of hemostasis. Intraluminal progression into thrombosis is efficiently blocked by natural antithrombotic enzymes and inhibitors, such as activated protein C, plasmin, and antithrombin. Thrombosis develops when the antithrombotic system fails to control further intravascular thrombin generation. Causality of macrovascular and/or microvascular thrombosis in morbidity and mortality has been directly documented in various diseases that include deep vein thrombosis, pulmonary thromboembolism, peripheral artery thrombosis and embolism, retina vein thrombosis, as well as myocardial infarction (Meadows (1965) Med. J. Aust. 4:409-411; Harland (1966) Lancet 26:1158-1160), ischemic stroke (Carmon (1966) J. Neural. Sci. 4:111-119), anthrax sepsis and meningococcal sepsis (Dalldorf (1977) Arch. Pathol. Lab. Med. 101:6-9), or heparin-induced thrombocytopenia (Rhodes (1973) Surg. Gynecol. Obstet. 136:409-416). Evidence for organ damage of thrombotic occlusion and hypoxic origin is also available in other disease groups, such as hemorrhagic fevers (Dennis (1969) Br. J. Haematol. 17:455-462; Gear (1979) Rev. Infect. Dis. 1:571-591; Ignatiev (2000) Immunol. Lett. 71:131-140), diabetic angiopathy (Ishibashi (1981) Diabetes 30:601-606; Boeri (2001) Diabetes 50:1432-1439), kidney disease (Miller (1980) Kidney Int. 18:472-479; McCutcheon (1993) Lupus 2:99-103), and several other conditions.
Although thrombosis and hemostasis are not identical molecular processes, they are similar enough that antithrombotic drugs developed to date inadvertently target both. Thrombosis is treated with antiplatelet, profibrinolytic, and anticoagulant agents, yet most of these agents can completely block both thrombosis and hemostasis when administered at their maximally effective doses. Antithrombotic drugs either target the building blocks of thrombi (fibrin and platelets) or inhibit molecules (coagulation factors) and cells (platelets) from participating in the thrombus-forming process. It is widely believed among clinicians and researchers that if an antithrombotic agent is unable to block hemostasis it will not work in thrombosis.
One of the oldest anticoagulant antithrombotic agents, heparin, is still the most widely given injection in the world. Sufficiently high doses of heparin can achieve nearly 100% efficacy but only at the cost of paralyzing hemostasis at such doses. Unfortunately, newer antithrombotic agents, such as fractionated heparins or direct thrombin inhibitors agents do not fare much better. As a result, antithrombotic agents, especially anticoagulants and profibrinolytic agents, must be administered at less than their maximally efficacious doses, and thrombosis remains an under-treated disease. Introduction of new compounds that are based on the promise of improved efficacy but are unable to promise improvement of hemostatic safety is unjustifiable. To date, antithrombotic compounds have fallen short of promising improvement of safety. The ideal antithrombotic agent would anti-coagulate circulating blood without adversely affecting hemostasis.
Thus, there remains a pressing medical need for the development of safe yet efficacious agents that block intravascular thrombin generation without paralyzing hemostasis.